Electrochemistry is a powerful tool for many different types of commercial processes including, for example, separations, sensing, identification, forming elemental metals, energy generation and storage, catalysis, and the like. However, the electrochemistry of rare earth elements, the lanthanides and the actinides, is generally a more difficult topic in view of unique properties of lanthanides and actinides. Electrochemically, lanthanide analysis is limited by their standard potentials (in the range of −1.99 and −3.90 V v. NHE; Cotton, S., Lanthanide and Actinide Chemistry, Wiley: 2007; Vol. 27). These potentials fall outside of the potential window of common liquid electrochemical solvents (e.g., aqueous and organic). For example, in aqueous solutions at platinum, the potential window (in 1 M acid) is limited between +1.3 and −0.7 V vs NHE by solvent electrolysis (Bard, A. J.; Faulkner, L. R., Electrochemical Methods, Fundamentals, and Applications, 2nd Ed.; John Wiley, 2001). Previously, researchers have resorted to mercury drop electrodes or chemically modified carbon paste electrodes to look at lanthanide compounds (Schumacher et al., Rev. Anal. Chem., 2013, 32(2), 159-171). In many cases, moreover, electrochemistry is undertaken in less tractable and more costly solvent systems of molten salts and ionic liquids (Binnemans, Chem. Rev., 2007, 107(6), 2592-2614; Yamagata et al., J. Electrochem. Soc., 153(1), E5-E9 (2006). Also, because most properties of lanthanides and actinides (e.g., masses, ionic radii, oxidation states, and standard potentials) vary little across the row, actinide and especially lanthanide separations are difficult as they rely on numerous sequential extractions.
As a result, present lanthanide and actinide detection and separation methods are tedious, costly, and time-consuming. Despite these difficulties, lanthanides and actinides are commercially important, critical materials, so commercial need drives the development for new approaches for rare earth electrochemistry and separations. For example, lanthanide isotopes are produced during fission of 235U, most of which decays to a stable, nonradioactive mixture that includes lanthanide elements.
Clearly, a commercial need exists for better electrochemical methods for lanthanides and actinides including, for example, separations, detections and identifications, and also in technologically important reactions like the oxygen reduction reaction (ORR), critical in batteries and fuel cells, for example.
Some attempts to develop electrochemical methodologies for lanthanides and actinides in simple aprotic electrochemical solvents have been made with negative or limited results.
Parrish et al., Tetrahedron Letters, 42 (2001), 7767-7770 describes an experiment in which Sm or Yb triflate compounds are reported reduced in acetonitrile at an unmodified electrode. However, in reproducing these experiments, it was found that the waves identified as lanthanide triflates disappeared on sparging with nitrogen. The reported results could not be reproduced, and there is no description of modifying the electrode.
For aqueous solvent, Yuan et al., Anal. Letters, 39, 373-385 (2006) teaches about detection of Europium(III) with use of differential pulse stripping voltammetry in water with Nafion modified electrodes further modified with multi-wall carbon nanotubes for more sensitive detection. Toyoshima et al., Radiochem. Acta, 96, 323-326 (2008) describes a flow electrolytic cell in water.